The present invention relates to a biosensor which measures a specified ground substance contained in a body fluid of a human being and an animal.
A conventional known technology is a technology of measuring the numerical value information on the subject substance in the body fluid such as a blood, urine and an interstitial liquid, e.g., measuring continuously a glucose concentration (which is a so-called a blood glucose level) in an interstitial liquid of the examinee by employing an electrochemical sensor embedded in an abdomen or an arm of the examinee. The electrochemical sensor is a sensor capable of detecting a minute amount of electric current by making use of electrochemical reaction, and is suited to detecting a minute amount of chemical substance in which oxidation-reduction reaction occurs.
The electrochemical sensor for measuring the glucose concentration may involve using, in many cases, a biosensor which detects a subject substance in a sample by utilizing enzyme reaction in a way that immobilizes the enzyme to a sensor portion. This type of biosensor generally has a working electrode and a counter electrode, in which the enzyme (e.g., glucose oxidase) is immobilized on the working electrode. The glucose concentration may be measured based on a response current obtained when applying a voltage to between the working electrode and the counter electrode.
The glucose oxidase produces gluconic acid by selectively reacting on the glucose under an existence of oxygen. Then, the oxygen is reduced, while hydrogen peroxide proportional to a quantity of the glucose is generated. The hydrogen peroxide can be oxidized electrochemically easily and can be therefore measured by use of a pair of electrodes. Namely, the response current value can be obtained by electrical oxidizing the hydrogen peroxide generated by the enzyme reaction of the enzyme. Then, the glucose concentration can be calculated based on a sampling current obtained by periodically sampling the electric current from the response current values acquired continuously.
By the way, a living body fluid contains an interference substance that affects a measurement value of the response current for measuring the glucose described above as the case may be. Known interference substances are exemplified by ascorbic acid, erythrocyte and vitamin C. If the interference substance is contained in the living body fluid serving as a measurement target fluid of a glucose concentration, the response current includes a signal component derived from nonenzymatic reaction (nonspecific reaction) like reaction between the interference substance and the electrode and a signal component derived from the oxidation-reduction reaction based on the enzyme. In this case, it has possibility that the glucose concentration acquired based on the response current is remarkably different from an actual value.
In an observation of the response current, it is difficult to determine whether the response current contains the signal component derived from the nonspecific reaction or not. Such being the case, there is a conventional technology (e.g., Patent document 1) of forming an interference-substance removing film on an electrode of a sensor and preventing interference substances other than a biometric component from reacting on the electrode.
[Patent document 1] U.S. Pat. No. 5,356,786
[Patent document 2] U.S. Pat. No. 7,003,341
[Patent document 3] U.S. Pat. No. 7,190,988
[Patent document 4] U.S. Pat. No. 7,462,264
The technology of forming the interference substance removing film disclosed in Patent document 1 has, however, a complicated configuration to dispose a reagent on the electrode. Further, the interference substance removing film can remove only an intended specified interference substance.